The iPhone 4 has an excellent on-screen keyboard, but if you must use a hardware keyboard, Brando now offers a Bluetooth keyboard for the iPhone 4 that's integrated into a case for the iPhone 4. Check it out:

iPhone 4 Case with Bluetooth Keyboard protective slim lined leather case for your iPhone 4 with keyboard built-in. This is made of fine genuine leather. The leather case offers high protection for your device. It features a slim, perfectly fit and stylish design. This is a Bluetooth enabled keyboard. It's portable and tiny design can be used for travel, school, or any other working environment with a Bluetooth enabled iPhone 4.